Prime Minister Yingluck Shinawatra arrived in one of her biggest strongholds of support on Thursday as some of her backers in Thailand's north prepared to raise militias to respond to any attempt to overthrow her.

Ms. Yingluck, who hasn't been seen publicly in Bangkok in more than a week, arrived in her hometown, the northern city of Chiang Mai, to a rousing welcome. Hundreds of well-wishers urged the embattled prime minister to...